如何使用此链接列表实现？

Question

I am learning data structures and algorithms for C++ from Goodrich. They have given this LinkedList implementation. I understand the code but I am not but I am not able to use this in main class. How do I construct an instance and make insert, delete? For example I tried to create an instance of the class as follows:

StringLinkedList() L;

But it is showing the error: expected ";" before 'L

#include <iostream>
#include <string>

using namespace std;

class StringNode {
private:
    string elem;
    StringNode* next;

    friend class StringLinkedList;
};

class StringLinkedList{
public:
    StringLinkedList();
    ~StringLinkedList();
    bool empty() const;
    const string& front() const;
    void addFront(const string& e);
    void removeFront();
private:
    StringNode* head;
};

StringLinkedList::StringLinkedList()
    :head(NULL) {}
StringLinkedList::~StringLinkedList()
    {while (!empty()) removeFront();}
bool StringLinkedList::empty() const
    {return head==NULL;}
const string& StringLinkedList::front() const
    {return head->elem;}
void StringLinkedList::addFront(const string& e){
    StringNode* v = new StringNode;
    v->elem=e;
    v->next = head;
    head=v;
}
void StringLinkedList::removeFront(){
    StringNode* old=head;
    head = old->next;
    delete old;
}


int main () {
}

Answer 1

Brackets () indicate a function call. If you want to declare a variable the syntax is

Typename variable_name;

Optionally, you may need to pass parameters to a constructor

Typename variable_name(param);

In C++11 the uniform initialisation syntax allows you to use {} but I digress. Either way they come after the variable name. In your case, this works:

StringLinkedList L;

When you say

StringLinkedList() L;

the compiler sees a typename, then expects a variable name, but gets () before the name L (BTW - it might deserve a longer name) so decided you must be making a function call, which should end with a semicolon. But it doesn't, it ends with L; so you get

expected ";" before 'L

Answer 2

You can create an instance and add and remove items like this:

int main () {
    StringLinkedList list; // construct an instance
    list.addFront("foo");  // Add "foo"
    list.addFront("bar");  // Add "bar"
    list.removeFront();    // Remove "bar"
    // List is automatically deleted now
}

Answer 3

The method that you use for creation of object isn't correct i dont know what do you mean by

StringLinkedList() L;

the StringLinkedList() is a call for the constructor of the class and it can be used for creating objects of class but no need to specify a constructor call unless needed. You can use the constructor if you are creating object and also want to initialise data members of objects for its creation.

StringLinkedList L=StringLinkedList();

is the correct method but you need to write only

StringLinkedList L;

as it will automatically call the default constructor. The actual process happening is the constructor create a temporary object and then assign it to your object variable L; but it will be done automatically

I dont see any problem in declaring the objects of linked list class like this.

int main ()
{
    StringLinkedList s1;
    s1.addFront("hai");
    s1.addFront("dude");
    s1.removeFront();
    cout<<s1.front();
}